Plating catalyst and method

ABSTRACT

Stable zero-valent metal compositions and methods of making and using these compositions are provided. Such compositions are useful as catalysts for subsequent metallization of non-conductive substrates, and are particularly useful in the manufacture of electronic devices.

This application is a divisional of application Ser. No. 12/968,219,filed on Dec. 14, 2010 now U.S. Pat. No. 8,591,636.

The present invention generally relates to the field of electrolessmetal plating, and more specifically to the field of catalysts useful inelectroless metal plating of non-conductive substrates used in themanufacture of electronic devices.

Printed circuit boards include laminated non-conductive dielectricsubstrates that rely on drilled and plated through-holes to formconnections between opposite sides of the circuit board or betweeninnerlayers of the board. Electroless metal plating is well known forpreparing metallic coatings on surfaces. Electroless metal plating ofdielectric surfaces requires the prior deposition of a catalyst. Acommon method used to catalyze or activate laminated non-conductivedielectric substrates, prior to electroless plating, is to treat thesubstrate with an aqueous tin-palladium colloid in an acidic chloridemedium. The colloid includes a metallic palladium core surrounded by astabilizing layer of tin (II) ion complexes, such as SnCl₃ ⁻, which actas surface stabilizing groups to avoid agglomeration of the colloids insuspension.

In the activation process, the palladium-based colloid is adsorbed ontoan insulating substrate, such as epoxy or polyimide, to activateelectroless metal deposition, such as electroless copper deposition.While not wishing to be bound by theory, it is believed that thecatalyst particles play roles as carriers in the path of electrontransfer from reducing agents to metal ions in the plating bath.Although the performance of electroless plating is influenced by manyfactors, such as composition of the plating bath and choice of ligand,the activation step is the key factor for controlling the rate andmechanism of electroless deposition.

Although colloidal tin/palladium catalyst has been commercially used asan activator for electroless metal, particularly copper, deposition fordecades, it has many shortcomings such as its sensitivity to air andhigh cost. Along with the reduction in sizes and increase in performanceof electronic devices, the packing density of electronic circuits hasbecome higher and subsequently industry standards for quality haveincreased. As a result of greater demands on reliability and inparticular due to the high and fluctuating cost of palladium,alternative catalyst compositions or compositions that useless-expensive metals or that reduce or eliminate the use of preciousmetals are desired. The stability of the colloidal tin/palladiumcatalyst is also a concern. As mentioned above, the tin/palladiumcolloid is stabilized by a layer of tin (II) ions. The counterions mayprevent palladium from aggregating but the tin (II) ions are readilyoxidized to tin (IV), thus the colloid cannot maintain its colloidalstructure. This oxidation is promoted by increases in temperature and byagitation. If the amount of tin (II) is allowed to fall close to zero,then palladium particles grow in size, agglomerate and precipitate andplating stops.

Considerable effort has been made to find new and better electrolessplating catalysts. For example, because of the high cost of palladium,much of the effort has been directed toward the development of anon-noble metal catalysts, particularly towards the development of acolloidal copper catalyst. There has also been work directed to thedevelopment of a tin-free palladium catalyst since the stannous chlorideused to reduce the palladium is costly and the oxidized tin requires aseparate step of acceleration. However, such catalysts have not beenshown to be sufficiently active or reliable enough for through-holeplating. Furthermore, these catalysts are not sufficiently stable,typically become progressively less active upon standing and the changein activity renders such catalysts unreliable and impractical forcommercial use.

Stabilizing moieties for ionic palladium other than tin have beeninvestigated. For example, U.S. Pat. No. 4,248,632 discloses certainpyridine ligands as stabilizers for ionic metal catalysts, such as ionicpalladium (Pd²⁺). The ionic metal is reduced only after it is adsorbedon the surface of a non-conductive substrate. Other known stabilizingmoieties include polyvinyl pyrrolidone (PVP) and other polymers. PVPplays the role of a protecting and stabilizing agent. Metal-ligandmoieties where the ligand serves as an effective mechanism for anchoringpalladium (II) catalysts to substrates have been reported. Other metalcolloids, such as silver/palladium and copper/palladium, using ionicpalladium have also been reported. Although alternative catalysts to theconventional tin/palladium catalyst have been developed, these still useionic palladium and none of these alternatives have provided thenecessary stability, activity and adsorption to dielectric surfacesdemanded in the manufacture of electronic devices such as printedcircuit boards.

The present invention provides a composition including 0.5 to 100 ppm ofa zero-valent metal, a stabilizer compound and water; wherein thezero-valent metal is chosen from palladium, silver, cobalt, nickel,gold, copper and ruthenium; wherein the stabilizer compound is chosenfrom compounds of formula (I) and formula (II)

wherein R¹ and R⁵ are independently chosen from H, (C₁-C₆)alkyl,(CR⁶R⁶)_(a)Z and (CH═CH)Z; R² and R⁴ are independently chosen from H,(CR⁶R⁶)_(a)Z, HO(C₁-C₆)alkyl, R⁷R⁷N(C₁-C₆)alkyl and (CH═CH)Z; R³═H,(C₁-C₆)alkyl or NR⁷R⁷; each R⁶ is independently chosen from H and NR⁷R⁷;each R⁷ is independently chosen from H and (C₁-C₆)alkyl; each R⁸ ischosen from H, (C₁-C₆)alkyl and NHR⁷; each R⁹ is chosen from H andCO₂R⁶; Z═CO₂R⁷, C(O)NR⁷R⁷ or NHR⁷; a=0-6; wherein (i) at least one of R¹and R⁵ is (CR⁶R⁶)_(a)Z or (ii) R³ is NR⁷R⁷; and wherein at least one R⁸is NHR⁷.

The present invention also provides a method of preparing the abovecomposition including combining the stabilizer compound, water and awater-soluble metal salt and then adding a sufficient amount of areducing agent to form the zero-valent metal.

Also provided by the present invention is a method including: (a)providing a substrate having a plurality of through-holes; (b) applyingthe above composition to the surface of the through-holes; and then (c)electrolessly depositing a metal on the surface of the through-holes.

As used throughout this specification, the following abbreviations shallhave the following meanings, unless the context clearly indicatesotherwise: ca.=approximately; ° C.=degrees Celsius; g=gram;mg=milligram; L=liter; mL=milliliter; ppm=parts per million;μm=micron=micrometer; nm=nanometer; mm=millimeters; DI=deionized;T_(g)=glass transition temperature; R.T.=room temperature; andrpm=revolutions per minute. All amounts are percent by weight (“wt %”)and all ratios are molar ratios, unless otherwise noted. All numericalranges are inclusive and combinable in any order, except where it isclear that such numerical ranges are constrained to add up to 100%.

The term “through-holes” includes blind vias. Also as used throughoutthis specification, the term “plating” refers to electroless metalplating, unless the context clearly indicates otherwise. “Deposition”and “plating” are used interchangeably throughout this specification.The term “alkyl” includes linear, branched and cyclic alkyl. Likewise,the term “alkenyl” includes linear, branched and cyclic alkenyl.“Halide” includes fluoride, chloride, bromide, and iodide. The terms“printed circuit board” and “printed wiring board” are usedinterchangeably throughout this specification. The articles “a” and “an”refer to the singular and the plural.

The present compositions include a zero-valent metal, a stabilizercompound and water. Preferably, the zero-valent metal and stabilizingcompound are present in the composition as stable nanoparticles. Morepreferably, the present compositions are solutions including stablenanoparticles comprising the zero-valent metal and the stabilizingcompound.

The water used in the present compositions may be any type, such as tapwater or DI water. Suitable zero-valent metals are those useful ascatalysts for electroless metal plating, such as, but not limited to,palladium, silver, cobalt, nickel, gold, copper and ruthenium.Preferably, the zero-valent metal is chosen from palladium, silver,cobalt, nickel, copper and ruthenium, and more preferably palladium,silver, copper, cobalt and nickel. Palladium and silver are mostpreferred. Mixtures of zero-valent metals may be used, such as a mixtureof palladium and silver or a mixture of palladium and copper. Thezero-valent metal is present in the composition in an amount of 0.5 to100 ppm, based on the weight of the composition. Preferably, thezero-valent metal is present in the composition in an amount of 1 to 100ppm, more preferably from 1 to 75 ppm, still more preferably from 5 to75 ppm, even more preferably from 5 to 50 ppm, and most preferably from5 to 35 ppm.

Suitable stabilizer compounds for the zero-valent metal are chosen fromcompounds of formula (I) and formula (II)

wherein R¹ and R⁵ are independently chosen from H, (C₁-C₆)alkyl,(CR⁶R⁶)_(a)Z and (CH═CH)Z; R² and R⁴ are independently chosen from H,(CR⁶R⁶)_(a)Z, HO(C₁-C₆)alkyl, R⁷R⁷N(C₁-C₆)alkyl and (CH═CH)Z; R³═H,(C₁-C₆)alkyl or NR⁷R⁷; each R⁶ is independently chosen fromH(C₁-C₆)alkyl, and NR⁷R⁷; each R⁷ is independently chosen from H and(C₁-C₆)alkyl; each R⁸ is chosen from H, (C₁-C₆)alkyl and NHR⁷; each R⁹is chosen from H and CO₂R⁶; Z═CO₂R⁷, C(O)NR⁷R⁷ or NHR⁷; a=0-6; wherein(i) at least one of R¹ and R⁵ is (CR⁶R⁶)_(a)Z or (ii) R³ is NR⁷R⁷; andwherein at least one R⁸ is NHR⁷. Preferably, R¹ and R⁵ are independentlychosen from H, (C₁-C₃)alkyl, NHR⁷, CO₂H, C(O)NR⁷R⁷, (CH═CH)CO₂H and(CH═CH)C(O)NH₂, and more preferably R¹ and R⁵ are independently chosenfrom H, CH₃, NH₂, NHCH₃, CO₂H, C(O)NH₂, C(O)N(CH₃)₂, CH₂CO₂H, CH₂CO₂NH₂,(CH═CH)CO₂H and (CH═CH)C(O)NH₂. R² and R⁴ are preferably independentlychosen from H, CO₂R⁶, C(O)NH₂, C(O)N(CH₃)₂ and (CH═CH)Z. R³ ispreferably H, (C₁-C₃)alkyl or NR⁷R⁷, and more preferably H, CH₃, NH₂NHCH₃ or N(CH₃)₂. Preferably, each R⁶ is chosen from H, (C₁-C₃)alkyl andN(di(C₁-C₃)alkyl), and more preferably H, CH₃, NH₂ and N(CH₃)₂. Each R⁷is preferably independently chosen from H and (C₁-C₃)alkyl, and morepreferably H and CH₃. Each R⁸ is preferably chosen from H, (C₁-C₃)alkyland NHR⁶, and more preferably H, CH₃, NH₂ NHCH₃ or N(CH₃)₂. Thestabilizing compounds are generally commercially available, such as fromSigma-Aldrich (St. Louis, Mo.), or may be prepared by methods known inthe art. These compounds may be used as-is or may be further purified.

Particularly suitable stabilizer compounds include, but are not limitedto, 4-dimethylaminopyridine, 4-aminopyridine, 2-aminopyridine,4-(methylamino)pyridine, 2-(methylamino)pyridine,2-amino-4,6-dimethylpyridine, 2-dimethylamino-4,6-dimethylpyridine,4-diethylaminopyridine, 4-aminonicotinic acid, 2-aminonicotinic acid,nicontinamide, 2-aminonicotinamide, picolinamide, picolinic acid,4-aminopicolinic acid, 4-dimethylaminopicolinic acid,2-(pyridin-3-yl)-acetic acid, 2-amino-3-(pyridin-3-yl)-propionic acid,2-amino-3-(pyridin-2-yl)-propionic acid, 3-(pyridin-3-yl)-acrylic acid,3-(4-methylpyridin-2-yl)-acrylic acid, 3-(pyridin-3-yl)-acrylamide,3-aminopyrazine-2-carboxylic acid. Preferred stabilizer compoundsinclude 4-dimethylaminopyridine, 4-aminopyridine, 2-aminopyridine,2-amino-4,6-dimethylpyridine, 4-aminonicotinic acid, 2-aminonicotinicacid, 3-(pyridin-3-yl)-acrylic acid, 3-(4-methylpyridin-2-yl)-acrylicacid, and 3-aminopyrazine-2-carboxylic acid.

The present compositions contain the zero-valent metal and thestabilizing compound in a molar ratio of 1:1 to 1:20. Preferably, themolar ratio of the zero-valent metal to the stabilizing compound is from1:5 to 1:20, and more preferably from 1:10 to 1:20.

Optionally, the present compositions may contain one or more of variousadditives common in electroless plating catalyst compositions, such assurfactants, buffers, pH adjusting agents, solubility aids such asorganic solvents. Mixtures of various additives may be used, such as apH adjusting agent and a buffer. Any suitable surfactants may be used,including anionic, non-ionic, cationic and amphoteric surfactants. Suchsurfactants may be present in an amount of from 0 to 25 ppm, based onthe weight of the composition. When present, it is preferred that theamount of the surfactant is from 0.5 to 25 ppm and more preferably from1 to 10 ppm. Buffering agents which may be used include, but are notlimited to, carboxylic acids, such as citric acid, tartaric acid,succinic acid, malic acid, malonic acid, maleic acid, lactic acid,acetic acid and salts thereof; amines and salts thereof; and amino acidsand salts thereof; and inorganic acids, such as boric acid, and theirsalts, and inorganic bases such as sodium bicarbonate. Compounds whichmay be used to adjust the pH include, but are not limited to, alkalimetal hydroxides, such as sodium and potassium hydroxide, and acids suchas mineral acids. When used, the optional buffering agents and pHadjusting agents are used in amounts sufficient to adjust the pH to adesired range.

Typically, the present compositions have a pH of 6-14. Preferably, thepresent compositions are alkaline, that is they have a pH of from >7 to14, more preferably they have a pH of from 7.5 to 14, even morepreferably 7.5 to 10, and still more preferably 8 to 10.

The present compositions are stable aqueous solutions of nanoparticlesthat are useful to catalyze electroless metal deposition in themanufacture of electronic components. By “stable” is meant that noprecipitate formation is visually observed upon storage at 20° C. for 3months. Preferably, the present compositions show no precipitate after 6months, and more preferably after 1 year storage at 20° C. Thesenanoparticles may have a variety of particle sizes. If the particlesizes become too large, the compositions may not be stable, that is,precipitation may occur. Suitable average particle sizes may be from 1nm to 1 μm, preferably from 1 nm to 500 nm, more preferably from 1 nm to100 nm. Particle sizes may be determined by known techniques, such as bylight scattering or transmission electron microscopy.

The compositions of the present invention may be prepared by combiningthe stabilizer compound, water, a water-soluble metal salt and areducing agent. Preferably, the stabilizer compound, water, and thewater-soluble metal salt are combined and then a reducing agent isadded. The amount of reducing agent used is any amount sufficient toform the desired zero-valent metal. The stabilizer compound, water andwater-soluble metal salt may be added in any order. Typically, thewater-soluble salt is dissolved in an amount of water. This saltsolution is then added to an aqueous solution of the stabilizing agent.The mixture is then stirred, typically at room temperature (ca. 20° C.),and the pH is adjusted as needed. Typically, stir bar agitation may beused for small volumes, such as up to 200 mL. Homogenizers may be usedfor larger volumes. Typical mixing rates may be from 3000 to 25000 rpm.A POWERGEN™ 700 homogenizer by Fisher Scientific is an example of anapparatus which may be used. Next, the reducing agent is added to themixture and stirring is continued. When palladium is used as thezero-valent metal, the catalyst solution is typically brown to black incolor after reduction. Following reduction, it is believed that stablenanoparticles comprising the stabilizing agent and the zero-valent metalare formed.

A variety of metal salts may be used provided that such metal salts aresufficiently water-soluble. Suitable metal salts include metal halides,metal nitrates, metal nitrites, metal oxides, metal acetates, metalgluconates, metal fluoroborates, metal alkylsulfonates, metal sulfates,metal sulfites, metal thiosulfates, metal thiocyanates, and metalcyanides. Exemplary metal salts include, without limitation, palladiumchloride, palladium sodium chloride, palladium potassium chloride,palladium ammonium chloride, palladium sulfate, palladium nitrate,palladium acetate, palladium oxide, silver nitrate, silver oxide, cobaltacetate, cobalt chloride, cobalt nitrate, cobalt sulfate, nickelsulfate, nickel methanesulfonate, nickel acetate, nickel fluoroborate,gold chloride, potassium gold cyanide, gold sulfite, gold thiosulfate,gold thiocyanate, copper sulfate, copper gluconate, copper acetate,copper nitrate, ruthenium chloride, ruthenium porphyrins, and rutheniumoxide. The amount of metal salts used will vary depending on the watersolubility of the particular metal salt. For example, palladium saltsmay be used in amounts of 5 mg/L to 10 g/L, and preferably from 100 mg/Lto 5 g/L.

A variety of reducing agents my be used to form the presentcompositions. Suitable reducing agents include, but are not limited to,compounds such as boron hydride compounds, such as amineboranes, such asdimethylamine borane (DMAB), trimethylamine borane, isopropylamineboraneand morpholineborane, sodium borohydride and potassium borohydride,hypophosphorus acid, ammonium, lithium, sodium potassium and calciumsalts thereof, formaldehyde, hypophosphites, such as sodiumhypophosphite, hydrazine anhydride, carboxylic acids, such as formicacid and ascorbic acid, and reducing sugars, such as glucose, galactose,maltose, lactose, xylose and fructose. The amount of reducing agent useddepends upon the amount of the metal salt in the composition. Typically,the reducing agents may be used in amounts of 5 mg/L to 500 mg/L,preferably in amounts of 20 mg/L to 200 mg/L.

Because the catalyst compositions contain a zero-valent metal, such asPd⁰, processes using these compositions avoid the need for a reducingstep prior to electroless metal plating. In addition, the compositionsenable good adhesion of metal to substrates. The compositions aretin-free, thus avoiding the problems associated with tin (II) ionsreadily oxidizing to tin (IV) and disrupting the catalyst. The problemwith ionic palladium particles growing in size and agglomerating andprecipitating is also greatly reduced and preferably avoided altogether.Since tin is excluded from the composition, the cost of the catalystcomposition is reduced because costly stannous chloride is no longerrequired. Further, the acceleration step required when using tin isavoided in preparation of a substrate for metallization, therebyeliminating a conventional step in the preparation of non-conductivesubstrates for metallization.

Compositions of the invention may be used as catalysts in electrolessmetal plating of substrates which include inorganic and organicmaterials such as glass, ceramics, porcelain, resins, paper, cloth, andcombinations thereof. Substrates also include metal-clad and uncladmaterials, such as printed circuit boards. Such printed circuit boardsinclude metal-clad and unclad substrates with thermosetting resins,thermoplastic resins and combinations thereof, and may further includefibers, such as fiberglass, and impregnated embodiments of theforegoing. The temperatures and time periods for the method steps formetallization of the substrates are conventional and are well known inthe art.

Thermoplastic resins include, but are not limited to: acetal resins;acrylics such as methyl acrylate, methyl methacrylate, ethyl acrylate,ethyl methacrylate, butyl acrylate and copolymers containing any of theforegoing; cellulosic resins such as cellulose propionate, celluloseacetate butyrate and cellulose nitrate; polyethers; nylon; polyethylene;polystyrene; styrene blends such as acrylonitrile styrene and copolymersand acrylonitrile-butadiene styrene copolymers; polycarbonates;polychlorotrifluoroethylene; and vinylpolymers and copolymers such asvinyl acetate, vinyl alcohol, vinyl butyral, vinyl chloride, vinylchloride-acetate copolymer, vinylidene chloride and vinyl formal.

Thermosetting resins include, but are not limited to, allyl phthalate,furane, melamine-formaldehyde, phenol-formaldehyde and phenol-furfuralcopolymers, alone or compounded with butadiene acrylonitrile copolymersor acrylonitrile-butadiene-styrene copolymers, polyacrylic esters,silicones, urea formaldehydes, epoxy resins, allyl resins, glycerylphthalates and polyesters.

The present compositions may be used to catalyze both low and high T_(g)resins. Low T_(g) resins have a T_(g) below 160° C. and high T_(g)resins have a T_(g) of 160° C. and above. Typically, high T_(g) resinshave a T_(g) of 160° C. to 280° C. or such as from 170° C. to 240° C.High T_(g) polymer resins include, but are not limited to,polytetrafluoroethylene (“PTFE”) and PTFE blends. Exemplary blendsinclude PTFE with polypheneylene oxides and cyanate esters. Otherclasses of polymer resins which include high T_(g) resins are epoxyresins, such as difunctional and multifunctional epoxy resins,bimaleimide/triazine and epoxy resins (BT epoxy), epoxy/polyphenyleneoxide resins, acrylonitrile butadienestyrene, polycarbonates (PC),polyphenylene oxides (PPO), polypheneylene ethers (PPE), polyphenylenesulfides (PPS), polysulfones (PS), polyamides, polyesters such aspolyethyleneterephthalate (PET) and polybutyleneterephthalate (PBT),polyetherketones (PEEK), liquid crystal polymers, polyurethanes,polyetherimides, epoxies and composites thereof.

In one embodiment, the present compositions may be used to deposit thezero-valent metal on the walls of through-holes. These compositions maybe used in both horizontal and vertical processes of manufacturingprinted circuit boards.

Through-holes are generally formed in a printed circuit board bydrilling or punching or any other method known in the art. After theformation of the through-holes, the boards are optionally rinsed withwater and a conventional organic solution is used to clean and degreasethe board followed by desmearing the through-hole walls. Desmearing iswell-known in the art and typically desmearing of the through-holesbegins with application of a solvent swell.

Solvent swells are well-known in the art and conventional solvent swellsmay be used to desmear the through-holes. Such solvent swells, typicallyinclude, without limitation, glycol ethers and their associated etheracetates. Conventional amounts of glycol ethers and their associatedether acetates may be used. Examples of commercially available solventswells which may be used are CIRCUPOSIT™ conditioner 3302, CIRCUPOSIT™hole prep 3303 and CIRCUPOSIT™ hole prep 4120, all commerciallyavailable from Dow Electronic Materials, Marlborough, Mass.

Optionally, the through-holes are next rinsed with water. An oxidizer isthen typically applied to the through-holes. Suitable oxidizers include,but are not limited to, sulfuric acid, chromic acid, alkalinepermanganate or by plasma etching. Typically alkaline permanganate isused as the oxidizer. An example of a commercially available oxidizer isCIRCUPOSIT™ promoter 4130 commercially available from Dow ElectronicMaterials.

Optionally, the through-holes are rinsed again with water. A neutralizeris then typically applied to the through-holes to neutralize any acidresidues or basic residues left by the oxidizer. Conventionalneutralizers may be used. Typically, the neutralizer is an aqueousalkaline solution containing one or more amines or a solution of 3 wt %peroxide and 3 wt % sulfuric acid. Optionally, the through-holes arerinsed with water and the printed circuit boards are dried.

After the neutralization step, the substrate (such as a printed circuitboard having through-holes) may optionally be conditioned by applying analkaline conditioner to the substrate. Such alkaline conditionersinclude, but are not limited to, aqueous alkaline surfactant solutionscontaining one or more quaternary amines and polyamines and one or moresurfactants. While the surfactants used are conventionally cationicsurfactants, other surfactants, such as anionic, nonionic andamphoteric, may be used, as well as combinations of surfactants. Inaddition, pH adjusters or buffers also may be included in theconditioners. Typically, cationic surfactants are combined withnon-ionic surfactants. Surfactants may be present in the conditioners inamounts of 0.05 to 5 wt %, and preferably from 0.25 to 1 wt %. Suitablecommercially available alkaline conditioners include, but are notlimited to, CIRCUPOSIT™ conditioner 231, 813 and 860, each availablefrom Dow Electronic Materials. Optionally, the through-holes are rinsedwith water after conditioning.

Cationic surfactants include, but are not limited to,tetra-alkylammonium halides, alkylrimethylammonium halides, hydroxyethylalkyl imidazoline, alkylbenzalkonium halides, alkylamine acetates,alkylamine oleates and alkylaminoethyl glycine.

Non-ionic surfactants include, but are not limited to, aliphaticalcohols such as alcohol alkoxylates. Such aliphatic alcohols haveethylene oxide, propylene oxide, or combinations thereof, to produce acompound having a polyoxyethylene or polyoxypropylene chain within themolecule, i.e., a chain composed of recurring (—O—CH₂—CH₂—) groups, or achain composed of recurring (—O—CH₂—CH—CH₃) groups, or combinationsthereof. Typically such alcohol alkoxylates are alcohol ethoxylateshaving carbon chains of 7 to 15 carbons, linear or branched, and 4 to 20moles of ethoxylate, typically 5 to 40 moles of ethoxylate and moretypically 5 to 15 moles of ethoxylate. Many of such alcohol alkoxylatesare commercially available. Examples of commercially available alcoholalkoxylates are linear primary alcohol ethoxylates such as NEODOL 91-6,NEODOL 91-8 and NEODOL 91-9 (C₉-C₁₁ alcohols having an average of 6 to 9moles of ethylene oxide per mole of linear alcohol ethoxylate) andNEODOL 1-73B (C₁₁ alcohol with an average blend of 7 moles of ethyleneoxide per mole of linear primary alcohol ethoxylate), all commerciallyavailable from Shell Chemicals.

Anionic surfactants include, but are not limited to,alkylbenzenesulfonates, alkyl or alkoxy napthalenesulfonates,alkyldiphenyl ether sulfonates, alkyl ether sulfonates, alkylsulfuricesters, polyoxyethylene alkyl ether sulfuric esters, polyoxyethylenealkyl phenol ether sulfuric esters, higher alcohol phosphoricmonoesters, polyoxyalkylene alkyl ether phosphoric acids (phosphates)and alkyl sulfosuccinates.

Amphoteric surfactants include, but are not limited to,2-alkyl-N-carboxymethyl or ethyl-N-hydroxyethyl or methyl imidazoliumbetaines, 2-alkyl-N-carboxymethyl or ethyl-N-carboxymethyloxyethylimidazolium betaines, dimethylalkyl betaines, N-alkyl-β-aminopropionicacids or salts thereof and fatty acid amidopropyl dimethylaminoaceticacid betaines.

An advantage of the present compositions is that they can be used todeposit a composition containing a zero-valent metal on the surface ofthe substrate, and particularly on the surface of through-holes in aprinted circuit board, without the need for a prior conditioning step.This eliminates a conventional step in the preparation of non-conductivesubstrates for metallization.

The optional conditioning step is followed by micro-etching thethrough-holes. Conventional micro-etching compositions may be used.Micro-etching provides a micro-roughened copper surface on exposedcopper (for example, innerlayers) to enhance subsequent adhesion ofdeposited electroless and electroplated metal. Micro-etches include, butare not limited to, 60 g/L to 120 g/L sodium persulfate or sodium orpotassium oxymonopersulfate and sulfuric acid (2%) mixture, or asulfuric acid/hydrogen peroxide mixture. An example of a commerciallyavailable micro-etching composition is CIRCUPOSIT™ microetch 3330available from Dow Electronic Materials. Optionally, the through-holesare rinsed with water.

Optionally, a pre-dip is then applied to the micro-etched through-holes.Examples of pre-dips include 2% to 5% hydrochloric acid or an acidicsolution of 25 g/L to 75 g/L sodium chloride. Optionally, thethrough-holes are rinsed with cold water.

The composition of the present invention is then applied to thethrough-holes to function as a catalyst for electroless metaldeposition. The aqueous composition is applied to the through-holes attemperatures from room temperature (ca. 20° C.) to 50° C., typicallyfrom room temperature to 40° C. The through-holes optionally may berinsed with water after application of the catalyst.

The walls of the through-holes are then plated with a metal, such ascopper, using an electroless metal plating bath. Conventionalelectroless baths including immersion baths may be used. Such baths arewell known in the art. Typically the printed wiring board is placed inan electroless or immersion metal plating bath containing the metal ionsof the desired metal to be deposited on the walls of the through-holes.Metals which may be deposited on the walls of the through-holes include,but are not limited to, copper, nickel, gold, silver and copper/nickelalloys. A layer of gold or silver finish using immersion gold or silvermay also be deposited over a copper, copper/nickel or nickel deposit onthe walls of the through-holes. Preferably, copper, gold or silver isdeposited on the walls of the through-holes, and more Preferably copperis deposited on the walls of the through-holes.

After the metal is deposited on the walls of the through-holes, thethrough-holes are optionally rinsed with water. Optionally, anti-tarnishcompositions may be applied to the metal deposited on the walls of thethrough-holes. Conventional anti-tarnish compositions may be used. Anexample of an anti-tarnish composition is ANTI TARNISH™ 7130,commercially available from Dow Electronic Materials. The through-holesmay optionally be rinsed by a hot water rinse and then the boards may bedried.

After the through-holes are metal plated with the electroless orimmersion metal baths, the substrates may undergo further processing.Further processing may include conventional processing by photoimagingand further metal deposition on the substrates such as electrolyticmetal deposition of, for example, copper, copper alloys, tin and tinalloys. Conventional electrolytic metal baths may be used. Such bathsare well known in the art.

The present compositions form a stable aqueous solution of zero-valentmetal nanoparticles which may be used to catalyze electroless metaldeposition of non-conductive substrates, particularly substrates used inthe manufacture of electronic components. In addition, the compositionsenable good adhesion of metal to substrates. The compositions aretin-free, thus the problems associated with tin (II) ions readilyoxidizing to tin (IV) and disrupting the catalyst is avoided. Theproblem with zero-valent metal particles growing in size andagglomerating and precipitating is also greatly reduced, and preferablyeliminated. Since tin is excluded from the composition, the cost of thecatalyst is reduced because the costly stannous chloride is no longerrequired. Further, the present compositions may be applied to thesubstrate without the need for a prior conditioning step and theacceleration step required when using tin is also avoided in preparationof a substrate for metallization, thus two conventional steps can beeliminated in the preparation of substrates for metallization.

EXAMPLE 1

To a beaker containing 30 mL of DI water at room temperature (ca. 20°C.) was added 122 mg 4-aminopyridine to form a stabilizer solution. Thebeaker was put in a 50° C. water bath to completely dissolve the4-aminopyridine stabilizer. In another beaker, 38.2 mg Na₂PdCl₄ (Pd⁺²)was dissolved in ca. 10 mL of DI water at room temperature. Thestabilizer solution was added dropwise to the palladium salt solutionwith vigorous stirring. Stirring was continued for another 15-20 minutesafter the complete addition of the stabilizer solution, after which 12mg NaBH₄ dissolved in ca. 2 mL DI water was added to the mixture withvery strong agitation. The solution quickly changed to a dark browncolor, indicating the reduction of Pd⁺² to Pd⁰. The resulting4-Aminopyridine/palladium nanoparticle catalyst solution was stirredvigorously for another 30 minutes.

Accelerated Stability Test:

The catalyst solution was then placed in a 50° C. water bath for atleast 12 hours to further test its stability, after which time thecatalyst composition showed no observable precipitate or turbidity.

A working catalyst solution was prepared by diluting the catalystconcentration to 25 ppm with DI water, adjusting the pH to 9.6 withHCl/NaOH, and adding 3 g/L sodium bicarbonate as a buffer. The pH of thecatalyst solution was measured using an Accumet™ AB15 pH meter fromFisher Scientific.

EXAMPLE 2

The procedure of Example 1 was repeated except the 4-aminopyridinestabilizer was replaced with 4-dimethylaminopyridine. The concentrationsof the components in the resulting composition were: 4.29 g/L of4-dimethylaminopyridine; 0.96 g/L of Na₂PdCl₄, 0.3 g/L NaBH₄, and 3 g/Lsodium bicarbonate. The resulting 4-dimethylaminopyridine/palladiumnanoparticle catalyst composition was subjected to the acceleratedstability test of Example 1 and no precipitate or turbidity wasobserved.

EXAMPLE 3

The procedure of Example 1 was repeated except the 4-aminopyridinestabilizer was replaced with 2-aminopyridine. The concentrations of thecomponents in the resulting composition were: 1.5 g/L of2-aminopyridine; 0.96 g/L of Na₂PdCl₄, 0.3 g/L NaBH₄, and 3 g/L sodiumbicarbonate. The resulting 2-aminopyridine/palladium nanoparticlecatalyst composition was subjected to the accelerated stability test ofExample 1 and no precipitate or turbidity was observed.

EXAMPLE 4

The procedure of Example 1 was repeated except the 4-aminopyridinestabilizer was replaced with 2-amino-4,6-dimethylpyridine. Theconcentrations of the components in the resulting composition were: 1.75g/L of 2-amino-4,6-dimethylpyridine; 0.325 g/L of Na₂PdCl₄, 0.6 g/LNaBH₄, and 3 g/L sodium bicarbonate. The resulting2-Amino-4,6-dimethylpyridine/palladium nanoparticle catalyst compositionwas subjected to the accelerated stability test of Example 1 and noprecipitate or turbidity was observed.

EXAMPLE 5

The procedure of Example 1 was repeated except the 4-aminopyridinestabilizer was replaced with 2-aminonicotinic acid. The concentrationsof the components in the resulting composition were: 1 g/L of2-aminonictotinic acid; 0.96 g/L of Na₂PdCl₄, 0.4 g/L NaBH₄, and 3 g/Lsodium bicarbonate. The resulting 2-aminonictotinic acid/palladiumnanoparticle catalyst composition was subjected to the acceleratedstability test of Example 1 and no precipitate or turbidity wasobserved.

EXAMPLE 6

The procedure of Example 1 was repeated except the 4-aminopyridinestabilizer was replaced with 3-aminopyrazine-2-carboxylic acid. Theconcentrations of the components in the resulting composition were: 4.5g/L of 3-aminopyrazine-2-carboxylic acid; 0.96 g/L of Na₂PdCl₄, 0.4 g/LNaBH₄, and 3 g/L sodium bicarbonate. The resulting3-aminopyrazine-2-carboxylic acid/palladium nanoparticle catalystcomposition was subjected to the accelerated stability test of Example 1and no precipitate or turbidity was observed.

EXAMPLE 7

The procedure of Example 1 is repeated except that the followingcomponents are used. In the following table, “DMAB” refers todimethylamine borane.

Metal:Stabilier Stabilizer Compound Metal Salt Reducing Agent (mol:mol)2-Aminonicotinamide CuCl₂ DMAB 1:10 4-Dimethylaminopico- AgNO₃ H₂CO 1:15linic acid 4-Diethylaminopico- RuCl₃ DMAB 1:18 linic acid2-(Methylamino)pyridine Na₂PdCl₄ NaBH₄ 1:15 2-Amino-3-(pyridin-2- AgNO₃H₂CO 1:10 yl)-propionic acid 3-(4-methylpyridin-2- CoSO₄ NaBH₄ 1:20yl)-acrylic acid 3-(pyridin-3-yl)-acryl- PdSO₄ DMAB 1:12 amide

EXAMPLE 9

A bimetallic salt solution was prepared by dissolving 23.5 mg ofNa₂PdCl₄ in 20 mL DI water, and then adding 20 mg of CuSO₄ pentahydrateand 47 mg sodium citrate to the solution. The mixed salt solution colorturned green. In a separate beaker, 88.3 mg of 4-aminonicotic acidstabilizer was dissolved in 20 mL water and 0.1 N NaOH was used toadjust the solution pH to 9.5. The stabilizer solution was then added tothe mixed palladium/copper solution with stirring. After the additionwas complete, 0.1 N HCl/NaOH was again used to adjust pH to 9.2. Next,23.7 mg of NaBH₄ in ca. 2 mL DI water was added with very strongagitation. The solution quickly changed to a dark brown color. Theresulting catalyst solution was stirred for another 30 minutes. Thecatalyst solution was then placed in a 50° C. water bath for 12 hours tofurther test its stability. A working solution was prepared by dilutingthe catalyst concentration to 25 ppm, adjusting the pH to 9.6 withHCl/NaOH, and adding 1.85 g/L boric acid as a buffer.

EXAMPLE 10 Comparative

The procedure of Example 1 was repeated using the following compounds asstabilizers: 2-hydrazinylpyridine; 2-dimethylaminopyridine;3-aminopyridine; methylpyrazine-2-carboxylate;4-amino-2,5-dichloropyridine; and 2-(benzylamino)pyridine. None of thecatalyst compositions prepared using these compounds passed theaccelerated stability test of Example 1, either forming precipitate orbecoming turbid.

EXAMPLE 11

Various printed circuit boards (Nelco-6 epoxy/glass, NP-175 epoxy/glass,370T, FR-406, TU-752 epoxy/glass, SY-1141, and SY-1000-2 epoxy/glass)were metalized using an electroless copper plating bath according to thefollowing general procedure.

Multiple through-holes were drilled in each of the boards. The averagediameter of the through-holes was 1 mm. The through-holes of each boardwere then desmeared, prepared for electroless copper plating andelectrolessly plated with copper in a vertical process as follows:

-   -   1. Each board was treated with 240 liters of solvent swell for 7        minutes at 80° C. The solvent swell was an aqueous solution        containing 10% diethylene glycol mono butyl ether, and 35 g/L of        sodium hydroxide.    -   2. The boards were then rinsed with cold tap water for 4 minutes        at room temperature.    -   3. The through-holes in each board were then treated with 550        liters of an alkaline oxidizing agent of 50-60 g/L aqueous        alkaline permanganate at a pH of 12 for 10 minutes at 80° C.    -   4. The boards were rinsed with cold tap water for 4 minutes at        room temperature.    -   5. The though-holes in the boards were then treated with 180        liters of an aqueous neutralizer composed of 3 wt % hydrogen        peroxide and 3 wt % sulfuric acid at 50° C. for 5 minutes.    -   6. The boards were then rinsed with cold tap water for 4 min    -   7. Optionally, the board was then treated with CONDITIONER™ 860        alkaline conditioner which included a cationic surfactant and        buffer system to maintain a pH of around 11. The alkaline        conditioners are commercially available from Dow Electronic        Materials. Whether a conditioner was needed depended upon the        particular stable zero-valent metal nanoparticle catalyst        employed.    -   8. Boards that were conditioned in step 7 were then rinsed with        cold tap water for 4 minutes at room temperature.    -   9. The through-holes of each board were then micro-etched with        100 liters of an aqueous alkaline solution of 20 g/L ammonium        persulfate for 2 minutes at room temperature.    -   10. The boards were then rinsed with cold tap water for 4        minutes at room temperature.    -   11. A pre-dip of 5% concentrated hydrochloric acid was then        applied to the through-holes for 1 minute at room temperature.    -   12. The boards were then rinsed with cold tap water for 1 minute        at room temperature.    -   13. Certain boards had their through-holes primed for        electroless copper plating with 2 liters of a composition of the        invention for 5 minutes at 40° C. The concentration of the        zero-valent metal nanoparticles was 25 ppm. The pH of the        catalyst was generally between 9 and 10. Other boards were        primed with 2 liters of a conventional tin/palladium catalyst        with a palladium particle concentration of 25 ppm for 5 min. at        40° C. as a control. The conventional catalyst had the following        formulation: 1 g palladium chloride; 300 mL concentrated HCl;        1.5 g sodium stannate; 40 g tin chloride; and water to one        liter.    -   14. The boards were then rinsed with cold tap water for 2.5 min        at room temperature.    -   15. The walls of the through-holes of the boards were then        plated with electroless copper for 15 minutes at 36° C. The        electroless copper bath had the following formulation:

COMPONENT AMOUNT Copper sulfate Pentahydrate 2 g Formaldehyde 2.5 gSodium hydroxide 5 g Ethylene diamine tetraacetate (EDTA) 25 g Chlorideions 5 g 2,2-Dipyridyl 2 ppm Water To one liter

-   -   16. After electroless copper deposition, the boards were rinsed        with cold tap water for 4 min at room temperature.

The catalyst solutions prepared according to Examples 1-4 did notrequire the conditioning step. The catalyst solutions prepared accordingto Examples 5-7 did require the conditioning step.

Each board was sectioned laterally to expose the copper plated walls ofthe through-holes. Multiple lateral sections 1 mm thick were taken fromthe walls of the sectioned through-holes of each board to determine thethrough-hole wall coverage for the boards using the European BacklightGrading Scale. Section (1 mm) from each board were placed under anOlympus GX71 optical microscope of 50× magnification. The quality of thecopper deposit was determined by the amount of light that was observedunder the microscope. The backlight results showed that the catalystcompositions of the present invention were comparable to theconventional ionic tin/palladium (Sn/Pd) catalyst.

What is claimed is:
 1. A method comprising: (a) providing a substratehaving a plurality of through-holes; (b) applying a compositioncomprising 0.5 to 100 ppm of a zero-valent metal, a stabilizer compoundand water; wherein the zero-valent metal is selected from the groupconsisting of palladium, silver, cobalt, nickel, gold, copper andruthenium; wherein the stabilizer compound is a compound of formula (I)or formula (II)

wherein R¹ and R⁵ are independently selected from the group consistingof H, (C₁-C₆)alkyl, (CR⁶R⁶)_(a)Z and (CH═CH)Z; R² and R⁴ areindependently selected from the group consisting of H, (CR⁶R⁶)_(a)Z,HO(C₁-C₆)alkyl, R⁷R⁷N(C₁-C₆)alkyl and (CH═CH)Z; R³═H, (C₁-C₆)alkyl orNR⁷R⁷; each R⁶ is independently H or NR⁷R⁷; each R⁷ is independently Hor (C₁-C₆)alkyl; each R⁸ is selected from the group consisting of H,(C₁-C₆)alkyl and NHR⁷; each R⁹ is H or CO₂R⁶; Z═CO₂R⁷, C(O)NR⁷R⁷ orNHR⁷; a=0-6; wherein (i) at least one of R¹ and R⁵ is (CR⁶R⁶)_(a)Z or(ii) R³ is NR⁷R⁷; and wherein at least one R⁸ is NHR⁷ to the surface ofthe through-holes; and then (c) electrolessly depositing a metal on thesurface of the through-holes; wherein the composition has a pH offrom >7 to 14; and wherein the composition is free of precipitate for 3months upon storage at 20° C.
 2. The method of claim 1 wherein the pH is7.5 to
 14. 3. The method of claim 1 wherein the zero-valent metal andstabilizer compound are in a molar ratio of from 1:1 to 1:20.
 4. Themethod of claim 1 further comprising the step of contacting the surfaceof the through-holes with an oxidizing agent prior to step (b).
 5. Themethod of claim 1 further comprising the step of contacting the surfaceof the through-holes with a surfactant prior to step (b).
 6. The methodof claim 1 wherein the metal is selected from the group consisting ofcopper, nickel, gold, silver and copper/nickel alloys.
 7. The method ofclaim 1 further comprising a step of electrolytically depositing asecond metal onto the electrolessly deposited metal of step (c).
 8. Themethod of claim 6 wherein the second metal is selected from the groupconsisting of copper, copper alloys, tin and tin alloys.